Mean-field density of states of a small-world model and a jammed soft spheres model

We consider a class of random block matrix models in $d$ dimensions, $d ge 1$, motivated by the study of the vibrational density of states (DOS) of soft spheres near the isostatic point. The contact networks of average degree $Z = z_0 + zeta$ are represented by random $z_0$-regular graphs (only the circle graph in $d=1$ with $z_0=2$) to which Erdos-Renyi graphs having a small average degree $zeta$ are superimposed. In the case $d=1$, for $zeta$ small the shifted Kesten-McKay DOS with parameter $Z$ is a mean-field solution for the DOS. Numerical simulations in the $z_0=2$ model, which is the $k=1$ Newman-Watts small-world model, and in the $z_0=3$ model lead us to conjecture that for $zeta to 0$ the cumulative function of the DOS converges uniformly to that of the shifted Kesten-McKay DOS, in an interval $[0, omega_0]$, with $omega_0 < sqrt{z_0-1} + 1$. For $2 le d le 4$, we introduce a cutoff parameter $K_d le 0.5$ modeling sphere repulsion. The case $K_d=0$ is the random elastic network case, with the DOS close to the Marchenko-Pastur DOS with parameter $t=frac{Z}{d}$. For $K_d$ large the DOS is close for small $omega$ to the shifted Kesten-McKay DOS with parameter $t=frac{Z}{d}$; in the isostatic case the DOS has around $omega=0$ the expected plateau. The boson peak frequency in $d=3$ with $K_3$ large is close to the one found in molecular dynamics simulations for $Z=7$ and $8$.